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Substitution with Retention in Organoboranes and Utilization of the Phenomenon for a General Synthesis of Pure Enantiomers

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Substitution with Retention in Organoboranes and Utilization of the Phenomenon for a General Synthesis of Pure Enantiomers Proc. Indian Acad. Sci. (Chem. Sci.), Vol. 100, Nos 2 & 3, April 1988, pp. 119-142. Printed in India. Substitution with retention in organoboranes and utilization of the phenomenon for a general synthesis of pure enantiomers HERBERT C BROWN* and BAKTHAN SINGARAM H C Brown and R B Wetherill Laboratories of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA Abstract. Organoboranes, readily available via the hydroboration of unsaturated organic compounds, exhibit a remarkable versatility in their reactions. The boron atom in these organoboranes can be readily converted into a wide variety of organic groups under very mild conditions, providing simple versatile syntheses of organic compounds. Exploration of these substitution reactions reveal that, with rare exceptions, the organoboranes transfer the alkyl group to other elements of synthetic interest with complete retention of stereochemistry. Recently we have discovered a method of synthesizing essentially optically pure organoborane intermediates. These optically active alkyl groups attached to boron can also be transferred with complete retention of optical activity. Consequently, it is now possible to achieve by a rational synthesis the preparation of almost any optically active compound with a chiral center, either R- or S-, in essentially 100% enantiomeric excess. Keywords. Substitution with retention; organoboranes; synthesis of pure enantiomers; retention of stereochemistry. The ether-catalyzed addition of diborane to unsaturated organic molecules - the hydroboration reaction - made organoboranes readily available (Brown and Subba Rao 1957; Brown et al 1975). A systematic study of the scope and characteristics of the hydroboration reaction revealed that the addition of borane to unsaturated organic molecules is essentially quantitative and proceeds in an anti-Markonikov manner (1). HB (1) B J~ 99% 1% The reaction involves a cis-addition of the H-B bond (2). (2) * For correspondence. 119 120 Herbert C Brown and Bakthan Singaram The addition takes place preferentially from the less hindered side of the double bond (3). (3) B f~ > 99% < 1% No rearrangements of the carbon skeleton have been observed, even in molecules as labile as a-pinene (4). OH HB ~ [O] ,.~ (4) Most functional groups can tolerate the hydroboration reaction (5). RO2C,~ HB ~ RO2C~BC [O..__[[[[[[[Q~]RO2C~O H (5) Initially, the hydroboration reaction did not attract much attention as a synthetically useful reaction. After all, hydroboration produces organoboranes. At the time we started, only three things were really known about organoboranes: (1) they were oxidized by air; (2)they were stable to water; (3)they formed addition compounds with bases. Our research program has taken this exotic group of chemicals, diborane and organoboranes from unknown materials of little interest to importaflt reagents with major synthetic importance. Exploration of the chemistry of organoboranes, with emphasis on reactions of synthetic utility, has revealed their exceptional versatility (Brown et al 1975). An unexpected characteristic of these reactions of organoboranes is the fact that organoboranes transfer the alkyl group to essentially most of the other elements of synthetic and biological interest with complete maintenance of stereochemica! integrity. Typical transformations are indicated in figure 1. Protonolysis As mentioned earlier, the organoboranes are remarkably stable to water, aqueous bases, and aqueous mineral acids. However, they undergo protonolysis by carboxylic acid. Protonolysis of organoboranes proceeds with retention of configuration (Brown and Murray 1986). Thus one can take advantage of the unique properties of the hydroboration reaction to achieve stereospecific hyd- rogenations (figure 2) (Zweifel and Brown 1964). Protonolysis of organoboranes also makes possible the stereospecific synthesis of deuterium derivatives (figure 3). Substitution with retention in organoboranes 121 R C H=C H C H=C H R cis, cis cis, trans trans, trans etc. ROH R\ N R , co~ RD RCHO R R' \---/ , R C H2OH RNH 2 RC H2C H=C H2 RR'NH RC-C R' RCsCH RR'CHNH 2 RC H2COR' ] ' ~ RR'C H O H RC H2C O2Et RCOC=CR' RC H(C N) 2 RR~C O H Figure 1. Chart summarizing representative substitution reactions of organoboranes. 9H2-)3B ~H3 ! I tra~- Pin~e EtCO2H ~ A - cis- Pinane Figure 2. Hydroboration-protonolysis of/3-pinene to provide either cis- or trans-pinane as desired. Oxidation Similarly, oxidation of organoboranes with alkaline hydrogen peroxide produces the alcohols in essentially quantitative yield with complete retention of configura- tion (figure 4) (Brown and Zweifel 1961b). 122 Herbert C Brown and Bakthan Singaram ~)3 B EtCO2D= ~D BI~ ~ Figure 3. Hydroboration-protonolysis of norbornene to demonstrate protonolysis with retention. NaOH rk.)7 NaOH ~ " OH HB ~ H202 ~ B / NaOH OH \ Figure 4, Representative hydroborations-oxidations which establish retention in the oxidation stage. Amination Organoboranes are readily converted by chloramine or O-hydroxylamine sulfonic acid to primary amines (Rathke et al 1966). The use of dimethylalkylboranes, prepared from lithium dimethylborohydride, is especially effective in permitting essentially complete utilization of the a!kyi groups (Brown et al 1987a). Here also the reaction proceeds with retention (figure 5). The reaction of organoboranes, such as monoalkyldichioroboranes, with organic azides provides a convenient route to secondary amines (Brown et al 1973). This procedure provides a simple route to pure N-exo-norbornylaniline, corresponding to retention in the reaction of the exo-norbornyl boron moiety produced in the hydroboration (figure 6). Formylation In the presence of hydride reducing agents (MH), carbonylation of organoboranes proceeds readily to provide an intermediate which is oxidized to the aldehyde or hydrolyzed to the methylol derivative. The use of B-alkyl-9-BBN derivatives is Substitution with retention in organoboranes 123 ~"" BMe2 1. LiMe2BH2 2. Me3SiCI I NH2OSO3H ,, ~H2 BMe'2 ."NH2 1. H20 ~ 2. NaOH 3 so3. ~an$ Figure 5. Amination of organohorane intermediates with retention of configuration. ~ N3 ~H , H20 ~ N,. N ~'~ BC12 exo Ph Ph Figure 6. Synthesis of secondary amines with retention from organoborane intermedi- ates and organic azides. / KOH f ~'"~ OH - OMH J ,,,, Br ~. MH Figure 7. Conversion of R-B-9BBN via carbonylation into aldehydes or methylol derivatives with retention of configuration. especially effective in permitting essentially complete utilization of the alkyl group (Brown and Knights 1969; Brown et al 1979). The stereospecificity realized in the hydroboration reaction is retained during the reaction (figure 7) (Brown et al 1969b). 124 Herbert C Brown and Bakthan Singaram Aikylboronate esters are also converted into the corresponding aldehydes by successive treatment with methoxy(phenylthio)methyllithium (MPML), mercuric chloride and buffered hydrogen peroxide (Brown and Imai 1983). This reaction also proceeds with retention of configuration, as observed in other related reactions of organoboranes. Thus, the trans-geometry obtained by hydroboration of 1-methylcyclohexene is retained in the product (figure 8). Homologation Reaction of alkylboronic esters with dichloromethyllithium, followed by reduction of the intermediate with potassium triisopropoxyborohydride (KIPBH), gives the corresponding one-carbon homologated boronic esters. Oxidation provides methy- Iol derivatives (figure 9) (Brown et al 1985c). Ketone synthesis Yet another reaction of organoboranes that proceeds with retention of stereoche- mistry is the synthesis of ketones via the DCME reaction. Dialkyiborinic acid I. BHBr2 -" I MPML .....L _ 2. 1/~ - 2. HgCI2 HO OH ~I[01 ~"'~ LiAIH'I ~ ''''~ OTs-" 2."BMSCHOTsC1 ~ "" trans Figure 8. Transformationof the hydroborationproduct from l-methylcyclohexeneinto trans-2-methylcyclohexane carboxaldehyde with complete retention of configuration. 1. BHBr2 ~ ~ D [o] ~ o exo on~" ~ "o-J Figure 9. Homologationof boronic esters into the higherboronic esters with retention. Substitution with retention in organoboranes 125 esters, now readily available via hydroboration with chloroborane-ethyl etherate, react with a,a-dichloromethyl methyl ether (DCME), in the presence of sterically hindered base, to transfer the alkyl groups from boron to carbon under remarkable mild conditions. Oxidation of the intermediate provides the corresponding ketone (figure 10) (Carlson and Brown 1973). a-Aikylation a-Halo esters can be alkylated readily by organoboranes in the presence of suitable bases. This reaction is applicable to a wide variety of a-halo derivatives, such as ethyl bromoacetate, bromoacetone, chloroacetonitrile, etc (Brown et al 1969a). The use of B-alkyl-9-BBN derivatives provides more economical utilization of the organic groups. In this reaction also, the transfer of the alkyl group from boron to carbon proceeds with stereochemical integrity (figure 11) (Brown et al 1969b). Acetylene synthesis The ate complex formed by the reaction of an organoborane with lithium acetylide-ethylene diamine reacts with iodine at -78~ to give the corresponding HBCI2- OEt2~ ~" )2BC! IROH I. DCME "" 2. Base 3. lOl Figure 10. Ketone synthesis via DCME reaction procccds with retention. B J 1. tBuONa CO2Et 2. BrCH2CO2Et LiA1H ! I I I ~ 2. LiA1H4 Figure 11. a-Alkylation reaction of organoboranes proceeds with retention of stereo- chemistry. 126 Herbert C Brown and Bakthan Singaram terminal acetylene (Midland et al 1974). Application of the reaction to 1- methylcyclopentene
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